BOSTON – A cybersecurity firm said on Tuesday that it uncovered stolen credentials from some 360 million accounts that are available for sale on cyber black markets, though it is unsure where they came from or what they can be used to access.

The discovery could represent more of a risk to consumers and companies than stolen credit card data because of the chance the sets of user names and passwords could open the door to online bank accounts, corporate networks, health records and virtually any other type of computer system.

Alex Holden, chief information security officer of Hold Security LLC, said in an interview that his firm obtained the data over the past three weeks, meaning an unprecedented amount of stolen credentials is available for sale underground.

“The sheer volume is overwhelming,” said Holden, whose firm last year helped uncover a major data breach at Adobe Systems Inc in which tens of millions of records were stolen.

Holden said he believes the 360 million records were obtained in separate attacks, including one that yielded some 105 million records, which would make it the largest single credential breaches known to date.